Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique
Jo-Anne Wartho, Simon P. Kelley, Stephen C. Elphick, 2014. "Ar diffusion and solubility measurements in plagioclases using the ultra-violet laser depth-profiling technique", Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences, F. Jourdan, D. F. Mark, C. Verati
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We describe the first direct measurements of Ar diffusion and solubility in plagioclases using ultra-violet (UV) laser ablation depth-profiling and noble gas mass spectrometer analyses of experimentally treated (599–1000 °C, 50–200 MPa of Ar) crystal fragments of labradorite and oligoclase. Labradorite 40Ar gain diffusion profiles were measured, yielding an activation energy of 26.72±4.58 kcal mol−1 (118.0±19.16 kJ mol−1) and a frequency factor of 9.77×10−9 (+8.79×10−8, −8.79×10−9) cm2 s−1 (95% confidence). The Ar solubility in labradorite was measured yielding a value of <0.2 ppb bar−1, which is similar to or lower than many rock forming minerals.
The labradorite diffusion parameters indicate Ar closure temperatures of 211 °C for a spherical diffusion geometry, and 243 °C for a planar diffusion geometry (for 100 µm-diameter grains, with cooling rates of 10 °C Ma−1). The data indicate that labradorite is less Ar retentive than K-feldspar at low temperatures, but more Ar retentive than K-feldspar at high temperatures, corroborating previous work on plagioclase. The relatively slow Ar diffusion rates in labradorite at magmatic temperatures may explain the common observation of older ages in large plagioclase grains in acidic volcanic systems.
Details of the UV laser depth profiles obtained from the labradorite and oligoclase samples used in this study are available at http://www.geolsoc.org.uk/SUP18608.
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Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences
Decoding the complete history of Earth and our solar system requires the placing of the scattered pages of Earth history in a precise chronological order, and the 40Ar/39Ar dating technique is one of the most trusted dating techniques to do that. The 40Ar/39Ar method has been in use for more than 40 years, and has constantly evolved since then. The steady improvement of the technique is largely due to a better understanding of the K/Ar system, an appreciation of the subtleties of geological material and a continuous refinement of the analytical tools used for isotope extraction and counting. The 40Ar/39Ar method is also one of the most versatile techniques with countless applications in archaeology, tectonics, structural geology, orogenic processes and provenance studies, ore and petroleum genesis, volcanology, weathering processes and climate, and planetary geology. This volume is the first of its kind and covers methodological developments, modelling, data handling, and direct applications of the 40Ar/39Ar technique.